This invention relates to retorting processes for recovering product hydrocarbons from oil shale and other hydrocarbon-bearing solids. The invention most particularly relates to those oil shale retorting processes wherein coke on the retorted shale is combusted to provide heat energy.
Many methods for recovering oil from oil shale have been proposed, nearly all of which utilize some method of pyrolytic eduction commonly known as retorting. To be competitive with the production of oils from petroleum stocks, the principal difficulty to be overcome has been recovering essentially all heat value from carbonaceous material in the shale without incurring prohibitive expense or environmental damage. Since shale usually contains only about 20 to 80 gallons of oil per ton, only a limited proportion of which can be recovered as product oil or gas, economical retorting must utilize remaining heat energy contained in the shale to provide heat for pyrolitic eduction. However, sulfur emissions in flue gases released from the retorting process must be restricted to the low levels required by law while this goal is being attained.
It is known to retort oil shale by a technique of contacting up-flowing oilbearing solids with down-flowing gases in a vertical retort, and one such technique is disclosed in U.S. Pat. No. 3,361,644. To educe product vapors, the upward-moving bed of shale particles exchanges heat with a down-flowing, hydrocarbonaceous and oxygen-free eduction gas of high specific heat introduced into the top of the retort at about 950.degree. to 1200.degree. F. In the upper portion of the retort, the hot eduction gas educes hydrogen and hydrocarbonaceous vapors from the shale and, in the lower portion, preheats the ascending bed of particles to retorting temperatures. As preheating continues, the eduction gas steadily drops in temperature, condensing high boiling hydrocarbonaceous vapors into a raw shale oil product while leaving a product gas of relatively high BTU content. The shale oil and product gas are then separated, and a portion of the product gas, after being heated, is recycled to the top of the retort as the eduction gas.
Retorted shale contains heat value in the form of coke, and many retorting processes pass retorted shale particulates through a combustion zone to combust the coke and thus recover heat energy. Because flue gases from combustion zones associated with shale retorts are usually at high temperature, many retorting processes recover heat therefrom. For example, as taught in U.S. Pat. No. 4,069,132, the hot flue gases may be utilized to exchange heat indirectly with boiler feedwater to generate process steam.
Maximizing the amount of heat energy recovered from oil shale by combusting coke on the retorted shale requires complete combustion. However, the difficulty of fully combusting coke deeply embedded in the relatively large-sized particles obtained from a retort similar to that disclosed in U.S. Pat. No. 4,069,132 has hampered the development of such a process. One can, of course, crush the shale in order to expose more coke for combustion, but retorting finely crushed shale generates unacceptable large pressure drops in the retort. On the other hand, crushing the shale subsequent to retorting poses twin problems of efficiently transporting and combusting small-sized particulate matter, which tends on the one hand to pack and plug during transport and on the other hand to require combustion as a fluidized bed.
Combustion of retorted shale to recover heat energy poses the additional difficulty of controlling the emission of pollutants from the combustor. Because retorted shale generally contains nitrogen and sulfur components, less than complete combustion of the coke generates carbon monoxide, ammonia, and hydrogen sulfide gases, the latter of which must be removed from flue gases by means of costly sulfur recovery processes. On the other hand, complete combustion may result in flue gases containing unacceptable amounts of nitrogen and sulfur oxide gases.
To solve the problem of sulfur dioxide production during complete combustion, U.S. Pat. No. 4,069,132 discloses a combustion process wherein the sulfur dioxide generated during the combustion of coke on the retorted shale is converted to stable inorganic salts by reaction with alkaline ingredients of the shale. This process utilizes a combustor through which hot retorted shale gravitates co-currently with air for combustion diluted by sufficient flue gas to control peak combustion temperature below 1670.degree. F. Under such conditions, the discharge of sulfur dioxide from the combustor is disclosed to be greatly minimized.
While the foregoing have met with some success, the need exists for further developments in shale retorting technology. In particular, a need exists for an oil shale retorting process for combusting crushed particles of retorted shale in a staged bed fluidized combustor for maximizing the amount of coke combusted while simultaneously minimizing the amounts of carbon monoxide and oxides of nitrogen and sulfur discharged to the atmosphere during said combustion.
Accordingly, a principal object of this invention is to provide an oil shale retorting process employing a two-stage combustor comprising a primary fluidized bed stage of combustion for maximizing recovery of heat energy from the retorted shale and a secondary overbed stage of combustion for minimizing the noxious gases emitted from the combustor.
Another object of the invention is to provide an oil shale retorting process employing recirculation of decarbonized fines to the combustor to aid in regulating temperature in the combustor and thereby minimize the amount of sulfur and nitrogen oxides discharged to the atmosphere during said combustion.
It is yet another object to provide an oil shale retorting process employing an ammonia sensitive control system for regulating the amount of oxygen fed into the dense bed combustion zone of said staged bed fluidized combustor.
These and other objects of the invention will be apparent from the following description taken in conjunction with the drawing.